Apart from the molecular state, the compact multi-quark and hybrid mesons are the other popular interpretations of the exotic states.
Of particular interest is the hybrid meson that is predicted by Quantum chromodynamics (QCD). This is because a light hybrid meson may
be composed of one quark and one antiquark together with a few gluons, which leads to the hybrid mesons being a nice place to understand
the interaction between the gluon and the quark. However, there are only two experimental candidates for light hybrid mesons, named \(\pi _{1}(1400)\) 
and \(\pi _1(1600)\) , have been recorded in the latest particle data group [1]}. Thus, more studies on the hybrid mesons
are needed.
